Here's a great video - a bit long - but excellent at explaining all the things you would need to do to change a photo. He shows you the basics of correcting a face or object, how to cut it out and put it on another layer and how to use the various NEW tools to quickly adjust the image, mask or lighting.
If you've used Photoshop before, please still watch this video as there are a range of NEW tools that only came out in CS5 and CS6 so unless you've used a copy of PS bought in the last year, then you will be missing out if you don't watch and learn - it includes Quick Select with expand, content aware (marvellous!!) and refining edges.
Remember if you're working in school you will need to save all your files from and to the MEDIA drive (CGSSTORE1) and please don't save hundreds of different versions! (yrs10 and 11 please take note). Always save a
photoshop version (he explains this at the end) which is a file with the ending 'psd'. Last thing, he doesn't discuss much in the way of formats and opts for JPG. This format is okay for making your image small but TIFF is better for saving more detail (as is PNG) and both these formats will keep any
fonts you have that are not on other computers. You can also use Print>Save as PDF for this purpose.
There are photos available for you to practice on that are similar to the ones he uses in the Photoshop Tutorial files. Please practice what he shows you as you go along - you can always PAUSE and REPLAY the video!
Good luck - any problems, just ask :)